Method for Constructing Water Barriers and Coastal Protection

ABSTRACT

The invention relates to a method for constructing a water barrier. In the method a core of quarry run is arranged on the underwater bottom and provided with a protective layer of stones or concrete blocks, with the proviso that, at least at the position of the water barrier, the underwater bottom is raised using bottom material. The method is particularly suitable for constructing a water barrier for relatively deep water.

The invention relates to a method for constructing water barriers andcoastal protection, such as in particular breakwaters, and a waterbarrier obtained with the method.

Breakwaters comprise structures erected from relatively hard rock whichare arranged in the sea in order to protect the coastal area behind.This protection is brought about in that the breakwater absorbs asignificant part of the wave energy of the incoming waves, whereby thewave reaches the coastal area in attenuated form. The known rockfillbreakwater is generally erected from a core of quarry run with a heavierprotective layer on the outside, the so-called armour layer. This armourlayer generally comprises larger and heavier stone blocks than the corematerial. Concrete elements are also applied. The stone blocks have asomewhat irregular shape so that the protective layer is sufficientlyporous or permeable for the waves. Because the waves can partiallypenetrate the protective layer, the wave energy is attenuated.

In addition to attenuating waves, a breakwater protects against coastaldeterioration through erosion, caused by unfavourable sediment transportfrom the coastal area to sea. The breakwater interferes in the naturaltransport pattern of sediment such that erosion of the coastal area isprevented, or in any case reduced.

Owing to the protective action of a breakwater regular sandreplenishments along the eroded coastline are less necessary. A drawbackof the known water barrier or coastal protection, in particularbreakwater, is that the effect on the sediment transport along thecoastline depends on a large number of parameters, such as for instancethe length of the breakwater and/or the distance thereof from the coast.At non-optimal ratios problems regularly occur which can be traced backto unfavourable sediment transport. The known breakwater can thus forinstance cause the formation of bell-shaped sediment accumulations(so-called ‘salients’). The known breakwater is also susceptible toerosion in the vicinity of the foot of the breakwater, which underminesthe stability of the breakwater.

The present invention has for its object to provide a method with whichwater barriers, and in particular breakwaters, can be constructed,wherein at least an equivalent protection is provided as with the knownwater barrier and wherein the above stated problems can be at leastpartly obviated.

This object is achieved according to the invention by providing a methodfor constructing a water barrier of the above stated type, wherein acore of quarry run is arranged on the underwater bottom and is providedwith a protective layer of stones or concrete blocks, with the provisothat, at least at the position of the water barrier, the underwaterbottom is raised using bottom material. With the invented method morebottom material or sediment is supplied at the position of the waterbarrier. This is at first sight unfavourable because the overall amountof bottom material in the vicinity of the water barrier greatlyincreases and the chance of loss of stability and/or undesirablesediment transport therefore increases. This expectation is furtherreinforced by newly supplied bottom material being generally less stablethan bottom material that has already been present for a long time.Surprisingly however, it has been found that a water barrier obtainedwith the invented method is surprisingly stable and causes lessundesirable sediment transport than expected.

With the inventive method a water barrier is obtained with a reducedoverall height of core and protective layer relative to the known waterbarrier. A significant advantage hereof is that the invented waterbarrier can be obtained in more economic manner than the known waterbarrier. The materials for the core and the protective layer aregenerally expensive to purchase and are moreover often transported byroad, this also resulting in high costs. The water barrier obtained withthe invented method in any case uses a reduced volume of core materialrelative to the known water barrier at substantially the same waterbarrier level, thereby saving costs. Because the amount of materialrequired for a water barrier increases sharply along with the depth, theinvented method is particularly suitable for constructing a waterbarrier for relatively deep water, which is understood to mean waterwith a depth of at least 8 metres, more preferably at least 14 metres,and most preferably at least 20 metres.

Another advantage of the method according to the invention is that thesupply of a part of the material for the water barrier can take place insimple and rapid manner. Although supply of the bottom material requiredfor raising the underwater bottom can for instance take place bysupplying this bottom material, generally sand, substantially by road,it is recommended that the bottom material is dredged in the vicinity,and more preferably in the immediate vicinity of the water barrier to beconstructed. Dredging is a per se known technique and can for instancebe carried out using a trailing suction hopper dredger. This comprises adrag head which, together with a suction conduit, is lowered under waterat the rear of the trailing suction hopper dredger until it contacts thebottom under the influence of its own weight. Through the forwardmovement of the trailing suction hopper dredger the drag head is draggedover the bottom for dredging, wherein the soil is loosened and suctionedaway with water via the suction conduit. If desired, the suctionedbottom material can be transported immediately via a transport conduitto the desired location, more in particular to the vicinity of the waterbarrier to be constructed.

In a preferred embodiment the method according to the invention ischaracterized in that the underwater bottom is raised with bottommaterial prior to the core being arranged. A more efficient method ishereby obtained, which moreover provides a greater reliability. It isthus possible for instance to determine the height of the raisedunderwater bottom before the core is arranged. If desired, the raisedpart of the underwater bottom can also be compacted or treated in othermanner.

The underwater bottom can in principle be raised to any height accordingto the invention, wherein the stability of the water barrier remainsguaranteed. A water barrier consisting substantially wholly of bottommaterial will generally be too unstable, and cannot therefore fulfillits function properly. In a preferred embodiment of the method accordingto the invention the underwater bottom is raised to a height such thatthe average water depth at the position of the water barrier decreasesby 20 to 80%, more preferably by 30 to 70%, and most preferably by 40 to60%. The water barrier is found to produce the best results within thisrange.

It has been found that a particularly stable water barrier is obtainedwith a method wherein the underwater bottom is raised to a water depthwhich does not exceed 50% of the closure depth, more preferably does notexceed 75% of the closure depth, and most preferably does not exceed theclosure depth. The closure depth is a term known to the skilled person:when water depths are regularly measured away from the coastline, thereis found to be a minimum water depth above which the water depths do notchange through time. This depth is called the closure depth. The closuredepth can be determined experimentally or, in the context of the presentpatent application, can be derived in simple manner by the followingapproximate formula:

d₁=1.75 H_(s0.137)   (1)

wherein d₁ represents the closure depth and H_(s0.137) is the effectivewave height at the position of the water barrier. The effective waveheight is that wave height which is exceeded a maximum of 12 hours peryear and therefore has a maximum occurrence probability of 0.137%.

The method according to the invention is preferably performed in thatthe underwater bottom is raised to a level such that the volume ofmaterial required for core and protective layer decreases by 20 to 80%relative to a water barrier with non-raised underwater bottom, morepreferably by 30 to 70%, and most preferably by 40 to 60%. Staticallystable breakwaters, such as of the above described type, shouldgenerally deform only little under wave attack. A relatively heavyconstruction is therefore required. With the method according to theinvention a water barrier is obtained which is lighter per unit volumethan the known water barrier. It is surprising that this has hardly anyor no effect on the stability of the water barrier. An undefended waterbarrier of sand would not at first sight appear to be a very obvioussolution. Nature will exert its influence on such a water barrier,deform it and possibly even level it completely in the course of time.The water barrier according to the invention comprises a quantity ofweighting material (quarry run and the stones of the protective layer)which is lower than the known water barrier with the same water barrierheight. Nevertheless, the above stated problems hardly occur.

A further preferred embodiment of the method according to the inventionis characterized in that the raised underwater bottom is compacted atleast at the position of the water barrier. Compaction of the raisedwater bottom can for instance be performed by vibration or bypile-driving with a drop weight. It is also possible and advantageous toprovide the raised water bottom with grout columns for furtherstabilizing thereof and to make the water barrier better resistant toearthquakes.

The raised underwater bottom can in principle be compacted to anydesired saturated density. The saturated density is understood to meanthe density of a volume of material which is substantially whollysaturated with water. It has been found advantageous for the raisedunderwater bottom to be compacted to a saturated density lying between1.6 ton/m³ and 2.3 ton/m³, more preferably between 1.7 ton/m³ and 2.2ton/m³ and most preferably between 1.9 ton/m³ and 2.1 ton/m³.

It is further advantageous to characterize the method in that a filterlayer is arranged between the raised underwater bottom and the core.This measure enhances the geotechnical stability. Soil erosion at thetoe of the water barrier, in particular a breakwater, is hereby reduced.Migration of bottom material to the core is further countered. Bottommaterial is in principle more impermeable than quarry run. Migration maythus result in a decreasing porosity of the core. The breakwater actionhereby becomes less effective. The present embodiment variant at leastpartially avoids this. In some cases the filter layer also lets throughwater. A relatively permeable water barrier or breakwater has a highprobability of sanding up, certainly in sand-rich conditions: waves andcurrent supply sand which then fills the pores (in the core).

The water barrier according to the invention can further comprisemultiple filter layers, for instance in the form of a stone layerbetween the core and the protective layer. A gradual transition ishereby obtained between the relatively finer and coarser materials. Ifdesired, the water barrier can further be provided with a toeconstruction for the purpose of supporting the protective layer in thedirection of the inclination of the slope and with a crown wall on topof the water barrier so that it is possible to walk thereon.

The water barrier according to the invention, and in particular abreakwater according to the invention, can be built with its crest belowor above water. The height position of the crest of the breakwaterrelative to the average water level determines the amount of wave energywhich is allowed over the structure, as well as the degree to whichdiffraction will occur. The more wave energy is allowed through, thesmaller the wave loads on the breakwater become. Breakwaters with theircrest under water have the further advantage that they allow through a(large) amount of wave energy, whereby the load on the breakwaterdecreases.

Fully impermeable breakwaters reflect the wave energy or dissipate theenergy in a relatively small stone volume. The breakwater is heregenerally loaded more heavily. Such a breakwater is therefore preferablygiven a heavier form.

A further preferred embodiment of the method according to the inventionis characterized in that the underwater bottom is raised gradually fromthe natural level thereof up to a position in the vicinity of the waterbarrier, whereby the raising has an inclination. The present variant hasthe advantage that the waves already at least partially lose theirenergy well before the water barrier. Model tests have moreover shownthat sediment transport to the area between water barrier and coastlineoccurs to lesser extent. It is further advantageous here when the heightof the raising increases at a substantially constant inclination in thedirection of the water barrier. The best results are achieved when theangle of inclination to the horizontal direction lies between 1:2 and1:20, more preferably between 1:5 and 1:15, and most preferably between1:7 and 1:10. The underwater bottom is preferably raised gradually fromthe natural level thereof up to a position in the vicinity of the waterbarrier, whereby the raising has an average inclination but wherein thisaverage inclination comprises one or more horizontal parts. Thehorizontal parts preferably have a length which is about 2 to 3 timesthe raised water depth at that position.

In order to further support the favourable effect of the inclination, ina preferred embodiment of the method the inclination is provided with aplurality of water barriers which are positioned at a mutual distancefrom each other and which lie substantially in line with the waterbarrier. The crests of the multiple water barriers preferably lie atincreasing height in the direction of the water barrier, with theproviso that the crest heights of the water barriers do not exceed thatof the water barrier.

The invention will now be further elucidated with reference to theaccompanying figures, without otherwise being limited thereto. In thefigures:

FIG. 1 shows a schematic cross-section of a first embodiment of a waterbarrier according to the invention; and

FIG. 2 shows a schematic cross-section of a second embodiment of a waterbarrier according to the invention.

Referring to FIG. 1, a water barrier 1 embodied as breakwater is shownwhich is obtained using the method according to the invention. The shownembodiment of water barrier 1 is obtained by raising the existingunderwater bottom 3, at least at the position of water barrier 1, withbottom material 4 supplied from elsewhere and subsequently arranging acore of quarry run 2 thereon, for instance by pouring it onto thesupplied bottom material 4. The core of quarry run 2 is provided with anapproximately 2 m thick protective layer 5 constructed from stones 6. Afilter layer 8 of a granular material, with a stone size between 5 and75 mm, is arranged between the raised water bottom 4 and core 2. It islikewise possible to apply a geotextile with a typical thickness ofbetween 2 and 5 mm.

In the variant shown in FIG. 1 the top of the water barrier protrudesabove water surface 7, although it is also possible for the top to liebelow the water surface 7, for instance in order to conceal thebreakwater from view. An existing water bottom at 14 m below water level7 can for instance be raised to an average of for instance 5 m belowwater level 7 using the invented method. The underwater bottom is herethus raised to a height at which the water depth is reduced from −14 mto −5 m, therefore by about 65%. In the given example a prior art waterbarrier, which is arranged directly onto the existing underwater bottom,is at least approximately 14 m high. With the method according to theinvention the overall height of core 2 and protective layer 6 amountsapproximately to at least only 9 m, this corresponding to a heightreduction of about 65%.

It is advantageous when underwater bottom 3 is raised gradually from thenatural level thereof up to a position in the vicinity of water barrier1, whereby the height of raising 4 increases at a substantially constantinclination 9 in the direction of water barrier 1. In the shown examplethe angle of inclination 10 to the horizontal direction lies between1:10 and 1:20.

The raising 4 can extend over a relatively great distance on the seaside of water barrier 1 (the left-hand side in FIG. 1). In a preferredvariant (not shown) inclination 9 is provided with a plurality of waterbarriers which lie substantially in line with water barrier 1 and whichare obtained in the same way as water barrier 1. The crests of the waterbarriers herein lie at increasing height in the direction of waterbarrier 1 as seen from the sea side, wherein they do not however exceedthe crest height of water barrier 1.

Referring to FIG. 2, another embodiment of a water barrier 1 is shown,which is likewise obtained using the method according to the invention.The shown embodiment of water barrier 1 is obtained by raising theexisting underwater bottom 3, at least at the position of water barrier1, using bottom material 4 supplied from elsewhere and subsequentlyarranging a core of quarry run (2, 2 a, 2 b) thereon, the central part 2a of which consists of coarser rocks and the right-hand part of whichconsists partially of even coarser rocks 2 b. The core (2, 2 a, 2 b) ispoured onto the supplied bottom material 4 in a number of operations.The core (2, 2 a, 2 b) is then provided with a protective layer 5,approximately 2 m thick and constructed from stones 6. A filter layer 8with a thickness of 5 to 75 mm is arranged between the raised underwaterbottom 4 and core 2. The parts (2 a, 2 b) provide for a locallyincreased porosity of core layer (2, 2 a, 2 b), whereby the wave energyis better dissipated locally. This variant further has a raising 4 withtwo substantially horizontal terraces (9 a, 9 b) and two parts (9 c, 9d) which have an incline. Such a structure has a favourable influence onthe breakwater action and the stability of water barrier 1.

The invention is by no means limited to the above described exemplaryembodiments, and many variants are possible within the scope ofprotection of the appended claims.

1. Method for constructing a water barrier, wherein a core of quarry runis arranged on the underwater bottom and is provided with a protectivelayer of stones or concrete blocks, with the proviso that, at least atthe position of the water barrier, the underwater bottom is raised usingbottom material.
 2. Method as claimed in claim 1, characterized in thatthe underwater bottom is raised with bottom material prior to the corebeing arranged.
 3. Method as claimed in claim 1 or 2, characterized inthat the underwater bottom is raised to a height such that the averagewater depth at the position of the water barrier decreases by 20 to 80%.4. Method as claimed in any of the foregoing claims, characterized inthat the underwater bottom is raised to a water depth which does notexceed 50% of the closure depth.
 5. Method as claimed in claim 4,characterized in that the underwater bottom is raised to a water depthwhich does not exceed the closure depth.
 6. Method as claimed in any ofthe foregoing claims, characterized in that the underwater bottom israised to a level such that the volume of material required for core andprotective layer decreases by 20 to 80% relative to a water barrier withnon-raised underwater bottom.
 7. Method as claimed in any of theforegoing claims, characterized in that the raised underwater bottom iscompacted at least at the position of the water barrier.
 8. Method asclaimed in claim 7, characterized in that the raised underwater bottomis compacted to a saturated density lying between 1.6 ton/m³ and 2.3ton/m³.
 9. Method as claimed in any of the foregoing claims,characterized in that a filter layer is arranged between the raisedunderwater bottom and the core.
 10. Method as claimed in any of theforegoing claims, characterized in that the underwater bottom is raisedgradually from the natural level thereof up to a position in thevicinity of the water barrier, whereby the raising has an inclination.11. Method as claimed in claim 10, characterized in that the height ofthe raising increases at a substantially constant inclination in thedirection of the water barrier.
 12. Method as claimed in claim 11,characterized in that the angle of inclination to the horizontaldirection lies between 1:2 and 1:20, more preferably between 1:5 and1:15, and most preferably between 1:7 and 1:10.
 13. Method as claimed inany of the claims 1-9, characterized in that the underwater bottom israised gradually from the natural level thereof up to a position in thevicinity of the water barrier, whereby the raising has an averageinclination and wherein this average inclination comprises one or moresubstantially horizontal parts.
 14. Method as claimed in any of theclaims 10-13, characterized in that the inclination is provided with aplurality of water barriers which lie substantially in line with thewater barrier.
 15. Method as claimed in claim 14, characterized in thatthe crests of the multiple water barriers lie at increasing height, butdo not exceed the crest height of the water barrier.